The goal of this project is to identify and study genes involved in cell fate specification during Drosophila eye development. Recent studies have revealed a group of genes required for eye development: eyeless (ey), eyes absent (eya), sine oculis (so) and dachshund (dac). In addition, targeted misexpression of ey, eya or dac induces ectopic eye formation indicating that they are key regulators of retinal fate. Furthermore, evidence of cross-regulation, functional and physical interactions between ey, eya, so and dac suggests that they are components of a retinal specification network. Another likely component of this network is Rx (retinal homeobox). Rx encodes a paired-type homeodomain that is highly conserved and is expressed in insect and vertebrate retinal precursors. Knockout mutations demonstrate that vertebrate Rx is required for early steps in eye development. Thus, we propose that rx is required for Drosophila eye development and plays a role in retinal specification. In addition, as rx, ey and so expression patterns overlap within the eye-antennal primordium, we propose that there may be interactions among these genes and/or their products. First, we will further characterize the structure and expression patterns of rx. Second, to determine if rx is required for retinal development, we will generate loss-of-function rx mutations by P-element mutagenesis. Third, to determine if rx expression is sufficient to induce retinal development, we will generate gain-of-function phenotypes by misexpressing rx using the GAL4/UAS expression system. These loss- and gain-of-function phenotypes win be exploited to determine the relationships between rx and ey, eya, so and dac.